1. Field of the Invention
The present invention relates to a semiconductor laser capable of emitting light at short wavelengths with high output power.
2. Description of the Prior Art
In order to increase the power output from a semiconductor laser, it has been proposed to provide a so-called window structure in which the effective band gap adjacent to the resonator ends or mirror surfaces, from which laser light is emitted, is greater than that of the central region. For example, FIGS. 2a and 2b show a prior art semiconductor laser with a window structure made by using a Zn diffusion technique described in IEEE Journal of Quantum Electronics, Vol. QE-15 (1979), pp 775-781. This laser has a negative electrode 8, a positive electrode 9, an n-type GaAs substrate 13, an n-type Al.sub.0.3 Ga.sub.0.7 As clad layer 14, an n-type Al.sub.0.06 Ga.sub.0.94 As active layer 15, an n-type Al.sub.0.3 Ga.sub.0.7 As clad layer 16, a SiO.sub.2 insulating film 17, and a p-type diffusion region 18 and 19.
This laser is fabricated as follows. The clad layer 14, active layer 15, and clad layer 16 are grown on the substrate 13 in this order, and then the insulating film 17 is formedon the clad layer 16. The insulating film 17 is cleaved with regions adjaced to the resonator ends left to form a window or stripe. Then, Zn is diffused through the window to the active layer 15 to form p-type diffusion regions 18 and 19. As a result, the active layer 15 becomes n-type adjacent to the resonator ends and p-type in the center.
The operation of this laser will be described. The band gap of the p-type diffusion region 19 in the active layer 15 is smaller than that of the undoped or n-type region because of the band shrinkage effect. Consequently, the n-type active regions adjacent to the resonator ends become transparent for the light produced in the p-type active layer, reducing the absorption by the surface level at the resonator ends, which is a cause of laser damage, making high power output possible.
The wavelength of light emitted from such a semiconductor laser shifts to the longer side because of the p-type diffusion into part of the active layer 15. The common method of shortening the wavelength of emitted light is to increase the proportion of aluminum in the active layer of the AlGaAs laser. In this method, however, the deterioration of the crystal raises the threshold level, making it difficult to provide a laser that has a good crystalline property and short wavelength light emission.